The present invention is related to a microwave filter having means for electromagnetic interference shielding, and in particular to a filter adapted for use in a radio frequency circuitry located in a stage next to an antenna for a radio communication system, such as a mobile communication system, a portable communication system, satellite communication system, and IMT-2000, for passing signals having desired frequencies while removing signals having undesired frequencies.
Recently, it is requested to use a transmitting frequency band and a receiving frequency band in a more close relationship to more effectively use frequencies, so that a filter used in a radio frequency circuit of a communication system of the type as explained in the above is necessary to have more superior attenuation characteristics in a prohibition band having frequencies higher or lower than a pass-band. Furthermore, it is requested to miniaturize and lighten the microwave filter in step with the tendency of miniaturizing and lightening communication systems.
In general, a conventional filter is composed of a plurality of resonators which do not have a shield for electromagnetic interference shielding.
In order to solve problems involved in the conventional filter of the type as explained in the above, various researches have been made. The most representative researches are proceeded in a way to cover all surfaces with a conductive material with the exception of a certain portion, and to use a shield for electromagnetic interference shielding.
One representative example of the prior art in this regard is disclosed in U.S. Pat. No. 4,954,796, xe2x80x9cMultiple Resonator Dieletric Filterxe2x80x9d in the names of Michael F. Moutrie and Phillip J. Gordon.
In summary, the above patent discloses a radio frequency duplexer comprising a receiver filter and a transmitter filter which are separately manufactured and surface-mounted on a printed circuit board, wherein the receiver and transmitter filters comprise a plurality of resonators enclosed within a cylindrical dieletric block and internal surfaces of the filters are plated with a conductive material, and wherein one end of each resonator is connected to one end of the dielectric block and a certain part of the resonator is also plated. The receiver and transmitter filters have a construction for rendering their input and output terminals to be readily connected with input and output parts of the printed circuit board, and the construction includes a plurality of tabs for supporting the connection between the input and output terminals of the dielectric block and the board.
However, the invention disclosed in the above patent has a problem in that it is difficult to improve its performance to a desired level, since it should also perform a function for connecting a shield or a bracket to the input and output terminals of the filter and to the input and output parts of the printed circuit board. Another example of the prior arts is disclosed in U.S. Pat. No. 5,745,018, xe2x80x9cCeramic filter with a coplanar shieldxe2x80x9d issued in the name of Reddy R. Vangala.
In brief, this patent provides a microwave filter comprising a shield, wherein a prescribed spacing between an opening of a dielectric block and the shield is defined as a height of a protrusion connected to the shield and the shield does not have to be directly connected to a printed circuit board, whereby its surface-mounting area can be reduced. The shield is directly connected to the opening of the dielectric block. The upper part of the shield formed from a metallic material coated with a tin film is provided with a plurality of windows for frequency trimming and the protrusion is directly connected to a metalized pattern formed on the opening of the dielectric body.
Yet, another example of the prior art is disclosed in U.S. Pat. No. 5,278,527, xe2x80x9cDieletric filter and shield thereforxe2x80x9d issued in the names Robert Kenoun and Darioush Agahi Kesheh.
The dielectric filter assembly disclosed in this patent comprises a dielectric filter formed of a block of dielectric material and provided with at least one windows for frequency trimming, and an L-shaped shield formed of an electromagnetic wave-absorptive material and having a first sheet portion seated upon one of the side surfaces of the dielectric block and a second sheet portion positioned to cover the opening parts of the dielectric block. In addition, it is possible to fix the shield within a recess formed in the body of filter.
Filters disclosed in the lastly mentioned two patents are advantageous in that the prescribed spacing can be easily obtained. However, they have some problems in that frequency trimming is difficult to be performed once the shield is fixed to the dielectric block, and since, in particular, the shield is not sufficiently contacted to the ground surface of a printed circuit board, they does not have sufficient grounding capability and thus it is impossible to insure attenuation characteristics and standing wave ratios to be obtained by using the shield.
The present invention is conceived to solve the above problems in ther art. It is the object of the present invention to provide a microwave filter having a shield for electromagnetic interference shielding, wherein the filter is adapted for use in a radio frequency circuit located in a stage next to an antenna for a radio communication system, such as a mobile communication system, a portable communication system, satellite communication system, and IMT-2000, for passing signals having desired frequencies while removing signals having frequencies having undesired frequencies.
In order to achieve the foregoing, the embodiments of the present invention provide a microwave filter for electromagnetic interference shielding, comprising a block of dielectric material plated with a metal with the exception of one surface, a plurality of resonators contained within the dielectric block, and a shield fixed on the plated surface of the block, wherein the shield has at least an alignment window for securing the easy alignment of the dielectric block and the shield through a visual observation.
Preferably, the shield has a folded end remote from the part fixed to the dielectric block, so that the shield can be easily grounded to a ground of a printed circuit board.
It is preferred that the surface of the shield fixed on the dielectric block is folded backward, so that the frequency trimming can be easily performed even after the shield is fixed on the dielectric block.
It is also preferred that the shield is formed with an embossed part, by which projections are formed on the inside of the shield while grooves are formed on the outside of the shield, so that the easy alignment, through a visual observation, of the dielectric block and the shield can be secured and a required spacing between them can be obtained.
The above and other objects and advantages will be more fully understand by a skilled person in the art from the explanation of embodiments to be described below with reference to the accompanying drawings.